A method and a system for controlling vehicle lane holding

ABSTRACT

The present invention relates to a method for controlling vehicle lane holding for a vehicle with an electric power assisted steering by means of a steering system ( 100 ) with a steering assistance actuator and one or more controllable vehicle state actuators comprising measurement of at least one vehicle position input signal with using an on-board vision system for determination of a relative vehicle lane position in the form of a lane curvature, transformation of the relative vehicle lane position to a target yaw and/or lateral vehicle state, measuring at least one steering input signal, determination from said one or more measured steering input signals a torque value applied by the driver via a steering wheel ( 120 ), transformation of said torque value to a relative to the afore-mentioned target yaw and/or lateral vehicle state a driver target relative yaw and/or lateral vehicle state, adding said target yaw and/or lateral vehicle state and said driver target relative yaw and/or lateral vehicle state together, and using the resulting yaw and/or lateral vehicle state as a reference signal to one or more controllers for the mentioned control of the one or more vehicle state actuators.

TECHNICAL FIELD

The present invention relates to method for controlling vehicle laneholding for a vehicle with an electric power assisted steering inaccordance with the preamble of claim 1 and to a system for controllingthe vehicle lane holding of a vehicle with an electric power assistedsteering having the features of the first part of claim 17.

BACKGROUND

Lane Keeping Aid, Lane Keeping Support, or Auto Pilot, hereafterreferred to as LKA, is a vehicle functionality aiming for helping thedriver to keep a vehicle in a road lane. Depending on legislation anddevelopment maturity, the guiding principles may differ significantly,spanning from only helping the driver to follow the lane when the driveris detected to steer the vehicle, at least for a reasonable time span,to be holding the steering wheel and potentially also is active todifferent degree of autonomy, where the driver does not need to beactive at all.

Other documented LKA behaviours are functionalities that do not guidethe driver continuously, but only when the driver is about to drift outof lane.

Traditional LKA variants, as e.g. described above, are lacking a goodinteraction with the driver in a series of important situations:

-   -   The driver might not always want to drive in the middle of the        lane, but rather to one side of the lane. The reason for that        can be e.g. because the lane is very wide as it is for some        expressways and that the driver wants to pass or wants to let        someone else pass. This is especially common for trucks. Another        important aspect for heavy trucks is that the driver might want        to centre the trailer rather than the towing vehicle (as a        consequence of e.g. weak road sides). Other situations where the        driver often wants to position the vehicle out of lane centre        are when the lane has longitudinal ruts that the driver wants to        avoid. These ruts are often water-filled. There might also be        accidents or tyre shifting in the roadway. Yet another situation        is when there are road constructions, so that the driver wants        to increase the distance to e.g. barriers.    -   Another common situation where LKA lacks interaction with the        driver is when the driver wants to change lane. It is often so        that the LKA function needs to be turned off during the lane        change.    -   Yet another situation is when the driver wants to steer        continuously, but have the convenience of having low steering        efforts, i.e. the driver wants to have a guiding, effort        reducing functionality.

The consequence of the two afore-mentioned LKA shortcomings are that thedriver does not appreciate the LKA functionality.

SUMMARY

It is therefore an object of the present invention to provide a methodand a system respectively through which one or more of theabove-mentioned problems and shortcomings are overcome.

According to one aspect of the present invention, it is an objective toprovide a system and a method respectively allowing to control thevehicle both from the information from the traditional LKA as well asadd the ability to add a driver input in order to make the driver beingable to control the lane position.

It is a particular object to provide a system and a method respectivelyfor the driver to be able to control the vehicle lane trajectory of avehicle.

These objects are achieved through a method and a system respectively asinitially referred to having the features of the respective independentclaims.

Advantageous embodiments are given by the dependent claims, and arediscussed in the description.

Particularly, in an advantageous embodiment, a mixed control as referredto above is achieved as follows:

-   -   A vehicle position identification functionality identifies the        vehicle position in relation to the actual lane of the road by        information achieved from an on-board vision system, an on-board        lidar system, an on-board radar system, an on-board        tele-communication system or an on-board GPS system. The data        achieved is in the form of e.g. a lateral lane position, a        heading angle, a lane curvature and a lane curvature derivative        with respect to the spatial coordinate of the vehicle heading        direction. The LKA functionality further is used to calculate a        target vehicle path in relation to the afore-mentioned vehicle        position in relation to the actual lane. Thus, the target        vehicle path needed to control the vehicle according to the        afore-mentioned traditional LKA is available.    -   In parallel to this vehicle position identification, a driver        intended vehicle lane position identification is made. It is in        Birk (WO 2010144049 A1) shown that a driver is controlling the        lateral state of the vehicle by means of applying a torque to a        steering wheel. This torque is transformed into a target lateral        vehicle state relative to the above-mentioned target lateral        state.

With these two target vehicle paths, it is, in accordance to this aspectof the present invention, possible to add the two target vehicle pathsso that the lane can be followed by the driver in a way that reduces theeffort at the same time as the lane position within the lane or evenoutside the lane can be controlled by the driver.

Further embodiments are described in the detailed description as well asin the dependent claims.

It will be appreciated that features of the invention are susceptible tobeing combined in any combination without departing from the scope ofthe invention as defined by the accompanying claims.

Advantageous embodiments are given by the respective appended dependentclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will in the following be further described by way ofexample only, in a non-limiting manner, and with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic figure showing a steering system for powerassisted vehicle steering,

FIG. 2 is a schematic figure showing the relation between the vehiclestate and the steering-wheel torque,

FIG. 3 is a schematic figure showing a vehicle on a road with positionindicating variables, and

FIG. 4 is a schematic figure showing a control diagram for the controlof the lateral vehicle state.

Still other objects and features of the present invention will becomeapparent from the following detailed description considered inconjunction with the accompanying drawings. It is to be understood,however, that the drawings are designed solely for purposes ofillustration and not as a definition of the limits of the invention, forwhich reference should be made to the appended claims and thedescription as a whole. It should be further understood that thedrawings are not necessarily drawn to scale and that, unless otherwiseindicated, they are merely intended to conceptually illustrate thestructures and procedures described herein. The same reference numeralsare used for illustrating corresponding features in the differentdrawings.

DETAILED DESCRIPTION

FIG. 1 is a schematic figure of a steering system 100 for which theinventive concept can be implemented. In a power assisted steeringsystem of a vehicle, there is a linkage between the front axle roadwheels 127 and the steering wheel 120. The linkage consists of asteering rack 124 with associated tie rods 125 connected via a pinion122 to the steering column 121. The steering column 121 incorporates atorsion bar 128 with a torque sensor for measuring the steering torqueapplied by the driver. The assistance torque is actuated by a steeringassistance actuator, which consists of an assistance motor 115 and anECU 110. The control of the level of assistance actuation in thesteering assistance actuator is controlled by a control system in theECU.

FIG. 2 is a schematic figure showing the relation between a vehiclestate 210 on the abscissa and a steering-wheel torque 220 on theordinate. The solid line 230 corresponds to a reference relation betweenthe vehicle state and the steering-wheel torque. Dashed line 240corresponds to how a steering-wheel torque is used to achieve a workingpoint on the reference relation between the vehicle state and thesteering-wheel torque. Dashed line 250 corresponds to how a workingpoint on the reference relation between the vehicle state and thesteering-wheel torque is used to achieve a target lateral state. Asteering-wheel torque can thus, according to the present invention, bythe before-mentioned dashed lines 240 and 250 be transformed to a targetlateral state.

FIG. 3 is a schematic figure showing exemplary position indicatingvariables related to the vehicle position on or within the lane. Avehicle 480 is located in a lane, with a lateral lane position, Ay 310,from the centreline 330 of the lane and with a heading angle, Θ 320,from the tangent of the centreline of the lane. The centreline 330 ofthe lane has a lane curvature, κ, and a spatial lane curvaturederivative, κ′, with respect to the forward distance from the currentposition. The driver can have, as will be further described in theembodiments below, an intention to position the vehicle in a positionthat differs from a target centre line 350 of the lane with a deltalateral lane position δΔy 360.

FIG. 4 is a schematic figure showing a control scheme over severalcontrol steps according to the invention. A vehicle 480, with itsseveral subsystems, has at every time a number of states, where a stateis defined as a translational or rotational position, a velocity or anacceleration. These states are schematically represented by a dashedline 405. The vehicle 480 is equipped with a number of sensors 410 fordirect or indirect measurements of the one or more vehicle states.Different types of sensors can be used such as a torsion-bar torquesensor, a steering-wheel angle sensor, vehicle wheel speed sensors, avehicle yaw rate sensor, a vehicle lateral acceleration sensor or acluster of vehicle velocity, rotational speed sensors and on-boardvision systems. The sensed or measured value or values of the vehiclestate or states 405 is/are communicated to the control steps by the useof a signal bus 415, where a signal bus is a transmission path on whichsignals can be read and/or transmitted. For the control of the vehicle480, there are according to the invention two control paths (A and B),namely an LKA path A indicated by 410-420-460-470-480 and a drivertorque control path B indicated by 410-440-450-460-470-480.

The LKA path A comprises a relative vehicle position calculationfunction, or means, 420, e.g. software in an electronic control unit, amicroprocessor, where the on-board vision system sensor signals are usedto calculate a number of vehicle position signals, or vehicle positionindicating variables, at least a curvature, and in advantageousembodiments also a curvature derivative of the lane.

The driver torque control path B comprises a driver torque calculationfunction, or means, 440, e.g. comprising software of a control unit,e.g. a microprocessor, which can be programmed/made such that asteering-wheel torque in FIG. 2 is a measure of a torque applied by thedriver via a steering wheel.

The driver torque control path B also comprises a target relativevehicle state calculation function, means, 450 (software in a controlunit, a microcomputer) which is a mathematical function that for aspecific vehicle speed transforms a driver torque to a lateral vehiclestate; at least one of the following states or a linear combination ofone or more of the following states; vehicle yaw rate or acceleration,vehicle lateral speed or acceleration, vehicle curvature and vehiclebody sideslip angle. The lateral vehicle state may, unless it is aposition as discussed above, furthermore be integrated into a targetrelative vehicle lateral lane position (integrated once if it isproportional to the lateral velocity; integrated twice if it isproportional to a lateral acceleration).

The target lateral state vector obtained from the relative vehicleposition calculation function 420 of control path A and the target deltalateral state vector obtained from the relative vehicle positionfunction 450 of control path B are added together in an additionfunction or addition step (software) 460 forming a mixed control targetlateral state vector. At least the lane curvature, or, in alternative,advantageous embodiments also the curvature derivative of the lane, isused in a vehicle state controller 470 to achieve the target lateralstate of the vehicle 480 in a controlled manner.

For facilitating the reading of the description of advantageousembodiments, below a section containing definitions used in thefollowing description as well as some functional explanations.

Definitions

An on-board vision system is a vehicle position identificationfunctionality that identifies the vehicle position in relation to theactual lane of the road by information achieved from an on-board camerasystem, an on-board lidar system, an on-board radar system, an on-boardtelecommunication system and/or an on-board GPS system that also can belinked to map data so that the lane curvature and the lane curvaturederivative can be achieved also in that way and possibly sensor fusionof them. The data achieved is in the form of e.g. a lane curvature, andoptionally e.g. one or more of a lane curvature spatial derivative withrespect to the forward distance from the current position, a headingangle and a lateral lane position. The LKA functionality further is usedto calculate a target vehicle path in relation to the afore-mentionedvehicle position in relation to the actual lane. Thus, the targetvehicle path needed to control the vehicle according to theafore-mentioned traditional LKA is available. The lane curvature and thelane curvature derivative can be used to calculate a target lateralstate vector consisting of the elements for the look-ahead distancecomprising at least;

-   -   a target curvature, which is the curvature of the lane, and in        alternative embodiments it also consists of;    -   a target curvature derivative, which is the spatial curvature        derivative of the lane with respect to the forward distance from        the current position.

A steering position actuator is an actuator which can be used toinfluence one or more of the steering actuator states, such as the rearwheel steering angle, the individual steering angles of the wheels, theaxle braking torque or force, the wheel braking torque or force, thedriving torque or force on the individual axles, the driving torque orforce on the individual wheels, the camber angle on each axle, or thecamber angle on each wheel.

A state is defined as a translational or rotational position, a velocityor an acceleration, or from one or more of these states derived states,such as e.g. a vehicle slip angle, which is the angle between thevehicle local x-axis and the vehicle speed vector.

A signal bus is a transmission path on which signals can be read and/ortransmitted.

Steering feel is built of the sum of at least some of the followingbuilding blocks (one or more of):

-   -   a lateral acceleration feedback torque, which is a function of        the steering angle and the vehicle speed,    -   a tyre friction torque, which is a function of the steering        angle,    -   a steering system friction torque, which is a function of the        steering angle,    -   a damping torque, which is a function of the steering angular        speed, and    -   a returnability torque, which is a function of the steering        angle.

A lateral acceleration feedback torque is a torque felt by the driverthat corresponds to the lateral acceleration of the vehicle.

A tyre friction torque is the friction between the tyres and the road ora model of this friction.

A steering system friction or a friction torque is the friction of theparts of the linkage of the steering system or a model of this friction.

A damping torque occurs owing to damping of the tyres and the steeringsystem or a model of this damping.

A returnability torque comes from the geometry of the steering system ora model of the steering system.

These torque contributions can be vehicle speed dependent. The torquecontributions can also be calculated via mathematical models or sensedvia sensors in the vehicle or steering system.

A compensation torque is one of, or the sum of one or more of, theabove-mentioned tyre friction torque, the friction torque, the dampingtorque and the returnability torque. The parts of the compensationtorque are calculated from mathematical models of the different torqueparts.

The lateral acceleration torque is calculated from a bicycle model,which uses vehicle speed and steering angle as input, and give thelateral acceleration as output. The lateral acceleration feedback is afunction of the lateral acceleration calculated from the vehicle model.

The mathematical model of the tyre friction torque is a model of anangle or angular speed driven hysteresis. The mathematical model of thetyre also contains a relaxation part such that as the tyre rolls, thetorque of the hysteresis will have a relaxation length so that thehysteresis torque decreases with the rolling length of the tyre. Therelaxation can preferably be the well-known half-life exponential decayfunction. The model of the tyre friction is the combination of thehysteresis and the relaxation so that there e.g. can be an increaseowing to the hysteresis torque taking place at the same time as thetorque decreases owing to the relaxation. The resulting torque of themodel is then the sum of the two parts.

The mathematical model of the friction torque is a model of an angle orangular speed driven hysteresis. The maximum torque in the hysteresiscan be shaped by a function so that the maximum torque is different oncentre compared to off centre.

The mathematical model of the damping torque consists of a dampingconstant times an angular speed or translational speed, such as e.g. therack velocity, measured somewhere in the linkage between the road wheelsand the steering wheel. The damping constant can be such that thedamping has a blow-off, such that the damping constant decreases forgreat angular or translational speeds. The damping constant can bevehicle speed dependent as well as different for steering outwardscompared to inwards. The damping constant can also be a function of thesteering-wheel or torsion-bar torque.

A returnability torque is a vehicle speed dependent and steering-wheelangle dependent torque.

A driver torque is the torsion-bar torque compensated with acompensation torque as discussed above.

Controllability describes the ability of an external input to move theinternal state of a system from any initial state to any other finalstate in a finite time interval.

A vehicle state controller is here defined as a dynamic function forachieving a target state in a vehicle in a controlled manner.

A vehicle state actuator, is an actuator that when actuated influencesone or several vehicle states. Examples of vehicle state actuators arebrakes, engine, controllable four-wheel-drive clutches, controllabledifferentials, active dampers, electric or hydraulic wheel motors andelectrically or hydraulically driven axles.

An actuator is a mechanism or system that is operated by an ECU andconverts a source of energy, typically electric current, hydraulic fluidpressure, or pneumatic pressure, into a motion, force or torque.

A target value, reference value or request is a set point for theactuator that is achieved by the use of either a closed loop controllerand/or a feed-forward controller.

A vehicle model is a mathematical model that transforms a road-wheelangle or steering angle and a vehicle speed to a number of vehicle yawand/or lateral states, e.g. vehicle yaw rate and acceleration, vehiclelateral speed and acceleration, vehicle curvature and vehicle bodysideslip angle.

A transformation is defined as a mathematical function or lookup tablewith one input value used to produce one output value. That means that atransformation can be used, with its tuneable parameters, to create arelation between the input value and the output value with arbitrarytuneable shape. A transformation can have time-varying parameters thatare even dependent on other values, a so-called gain scheduling, so thatthe transformation is a function with parameters that themselves arefunctions. An example of such a transformation is a vehicle state todriver torque relation where the relation is a vehicle speed dependentcontinuously rising, degressive shaped function.

A transfer function is the relation of the outputs of a system to theinputs of said system, in the Laplace domain, with the variable s,considering its initial conditions. If we, as an example of a singleinput, single output system, have an input function of X(s), and anoutput function Y(s), the transfer function G(s) is here defined to beY(s)/X(s).

A steering-wheel torque measurement is a torque measured in the steeringcolumn or steering wheel or a force measured in the steering rack timesthe torque ratio between the steering rack and the steering wheel.

A steering-wheel angle is here referred to as any angle between thesteering wheel and the road wheel times the ratio between the angulardegree of freedom and the steering-wheel angular degree of freedom. Itcan also be a rack position times its ratio between the racktranslational degree of freedom to the steering-wheel angular degree offreedom.

A trailer arrangement is defined as a passenger car trailer or caravan,or for a heavy truck a full-trailer, supported by front and rear axle oraxles and pulled by a drawbar, a semi-trailer, or a dolly with asemi-trailer.

The steering angle, which is here shown for one wheel, but if the wheelsare steered differently, as in the case for e.g. Ackermann steering, thesteering angle is defined as the mean value of the angles of the twowheels.

A natural coordinate system or natural coordinates is another way ofrepresenting direction. It is based on the relative motion of the objectof interest, the vehicle, rather than a fixed coordinate plane (x, y).The unit vectors (t, n) are:

-   -   t is oriented parallel to the horizontal velocity at each point,    -   n is oriented perpendicular to the horizontal velocity and        pointing positively to the left.

A vector is an array of one or more elements.

A linear function f(x) is a function that satisfies additivity,f(x+y)=f(x)+f(y) and homogeneity of degree 1, f(αx)=αf(x) for all α. Thehomogeneity and additivity properties together are called thesuperposition principle, which implies that the order of lineartransformations is not important.

The purpose of this first embodiment is purely to increase theconvenience of the driver by making the effort to steer the vehiclelower. In this embodiment, the driver steers the vehicle, but thesteering feel is built around the curvature of the road, i.e. thenatural coordinates of the road. This is achieved by using the lanecurvature and curvature derivative only from control path A of FIG. 4and calculate a delta curvature in control path B and add the twotogether in the addition (460) to form the target curvature. Finally,the error between the target curvature and the actual is minimized bythe use of a controller (470). The result is that the drivers steeringefforts will be as low as if the road were straight. But the driver hasto steer himself in order to stay in lane. This first embodiment isdescribed in detail as well as in general terms, below.

According to one aspect of the present invention, as also referred toearlier in this application, it is an objective to control the vehicleboth from the information from a traditional LKA as well as add theability to add a driver input in order to make the driver being able tocontrol both the lane position as well as being able to change lane withreduced effort, but the driver has to steer himself in order to stay inlane, here called a mixed control. Such a mixed control is in a firstadvantageous embodiment achieved by a series of steps according to FIG.4:

In control path A, the following method steps are taken:

In the relative vehicle position calculation function (also calledmeans) 420, the sensor signals from the on-board vision system are,according to the definition above, used for:

-   -   Calculation of the curvature of the lane.    -   Calculation of the spatial lane curvature derivative with        respect to the forward distance from the current position.    -   These two measures of the vehicle states, lane curvature and        lane curvature derivative are measured and/or calculated for a        distance in front of the vehicle, a look-ahead distance.

The output of the relative vehicle position calculation function 420 ishere a target lateral state vector consisting of the elements for thelook-ahead distance comprising at least;

-   -   a target curvature, and in alternative embodiments it also        consists of    -   a target curvature derivative.

In control path B, to have normal steering feel around the statesdescribed by the target lateral state vector, the following method stepsare taken:

In the driver torque calculation function (also called means) 440, thesensor signals from the steering system are, according to the definitionabove, used for:

-   -   Calculation of a torque applied by the driver, a driver torque,        which as such is an indicative of the driver intention. This        driver torque is in one embodiment the torsion-bar torque        signal. In an alternative embodiment, the driver torque is the        torsion-bar torque compensated with a compensation torque, as        defined in the definitions part above.    -   In another alternative embodiment of the present invention, the        parts of the compensation torque are further compensated for the        target lateral state vector values so that the target lateral        state vector does not affect the steering feel. This is done by        transforming the target lateral states from the relative vehicle        position calculation 420 from the target curvature to a target        steering angle and from the target curvature derivative to a        target steering angular speed by the use of a vehicle model and        subtracting these values from the steering angle and steering        angular speed of the steering feel equation building blocks:        -   The tyre friction torque, which is a function of the            steering angle, is altered by subtracting the target            steering angle from the steering angle to a new steering            angle and then to use this new steering angle (instead of            the steering angle) as input to the tyre friction torque.        -   The steering system friction torque, which is a function of            the steering angle, is altered by subtracting the target            steering angle from the steering angle to a new steering            angle and then to use this new steering angle (instead of            the steering angle) as input to the steering system friction            torque.        -   The damping torque, which is a function of the steering            angular speed, is altered by subtracting the target steering            angular speed from the steering angular speed to a new            steering angular speed and then to use this new steering            angular speed (instead of the steering angular speed) as            input to the damping torque.        -   The returnability torque, which is a function of the            steering angle, is altered by subtracting the target            steering angle from the steering angle to a new steering            angle and then to use this new steering angle (instead of            the steering angle) as input to the returnability torque.

In the target relative vehicle state calculation function (also calledmeans) 450, the driver torque is, according to the definition above,used for a transformation of the driver torque, according to FIG. 2, toa target delta lateral state (e.g. a delta lane curvature, δκ, cf. FIG.3:s detailed description). This target delta lateral state is acurvature relative to the target trajectory calculated in the relativevehicle position calculation function 420 above, or in other words atarget delta curvature in the natural coordinates of the targettrajectory calculated in the relative vehicle position calculationfunction 420.

The output of the target relative vehicle state calculation function 450is a target delta lateral state vector consisting of the target deltacurvature and a derivative the target delta curvature.

After the two control paths, A and B, there is an addition performed inaddition function or addition step 460 where the target lateral statevector (e.g. κ, cf. FIG. 3:s detailed description) and the target deltalateral state vector (e.g. δκ, cf. FIG. 3:s detailed description) areadded together into a mixed control target lateral state vector. Theoutput of the addition in the addition 460 is a mixed control targetlateral state vector consisting of the elements for the look-aheaddistance comprising at least;

-   -   a mixed control target curvature, and in alternative embodiments        it also consists of    -   a mixed control target curvature derivative.

In the vehicle state controller 470, the vehicle path is controlled inthe following method steps:

-   -   Calculation of a target yaw and/or lateral vehicle state.    -   Controlling the vehicle actuators towards this target yaw and/or        lateral vehicle state.

In a first embodiment of the first method step of 470 for thecalculation of a target yaw and/or lateral vehicle state, the mixedcontrol target lateral state vector is used to calculate e.g. a targetlateral acceleration. This calculation is made by first calculating thecurvature function of the distance, which is the sum of the curvatureand the spatial curvature derivative with respect to the forwarddistance from the current position times the distance in the look-aheadframe of interest. With the vehicle speed, this function is converted toa function of time rather than of distance. The target lateralacceleration is the curvature function of the distance times the squareof the vehicle speed.

Besides the before-mentioned target vehicle lateral acceleration, otherpossible target vehicle states are curvature, road-wheel angle,steering-wheel angle, vehicle lateral velocity, vehicle slip angle andvehicle yaw rate or linear combinations thereof, or in other words, thetarget can be generalized to target yaw and/or lateral vehicle states.

In a first embodiment of the second method step of the step 470 for thecalculation of a target yaw and/or lateral vehicle state, whichcomprises controlling the vehicle actuators towards this target yawand/or lateral vehicle state, the target yaw and/or lateral vehiclestate function of time is fed through an inverse vehicle model. Thatmeans that, ideally, with vehicle speed and vehicle model, the targetvehicle actuator states will be such that the vehicle path will followthe target vehicle path.

In the case of front-wheel steering only, the target vehicle actuatorstate is the front-wheel angle. This angle is controlled in a typicalfront steering gear of the vehicle 480 with a road-wheel angle interfaceor a steering-wheel angle interface times the ratio to the road wheels.

In the case of front-wheel and rear-wheel steering, the target vehicleactuator states are the front-wheel and rear-wheel angles. These anglesare controlled in typical front and rear steering gears of the vehicle480 with a road-wheel angle interface or a steering-wheel angleinterface times the ratio to the road wheels and a rear-wheel angleinterface.

Or alternatively, the target lateral states as well as the target deltalateral states are transformed to any of the in the definition listedvehicle yaw and/or lateral states.

Note specifically that the transformation between different yaw and/orlateral vehicle states from the target lateral states that is the resultof the relative vehicle position calculation 420, the target deltalateral states that is the result of the target relative vehicle statecalculation 450 to the actuator states in the vehicle state controller470 can be in any order, meaning that:

The target lateral state vector that is the result of the relativevehicle position calculation 420, and the target delta lateral statevector that is the result of the target relative vehicle statecalculation 450 can be in the form of steering angles or any of the inthe definition listed vehicle yaw and/or lateral states, i.e. they canbe generalized to vehicle yaw and/or lateral vehicle states. Dependingof the choice of states, the transformation needed in the form of aquasi-static vehicle model, which for a person skilled in the art is thesame as an invertible function of steering angle and vehicle speed. Thatmeans that for every vehicle speed, there is a function between thesteering angle and any of the yaw and/or lateral vehicle states. Andhence, as the equations described in the different embodiments arelinear or can be linearized, the order of transformation is not ofimportance, which lies within the definition of the term linear.

With the two target vehicle paths, control path A and control path B,all combinations of alternatives, it is, in accordance to this aspect ofthe present invention, possible to add the two target vehicle paths toachieve a mixed control of the vehicle path so that the lane can befollowed at the same time as the lane position within the lane or evenoutside the lane can be controlled by the driver. The result is that thedrivers steering efforts will be as low as if the road were straight.

It should be clear that the invention is not limited to the specificallyillustrated embodiments but that it can be varied in a number of wayswithin the scope of the appended claims.

1. A method for controlling vehicle lane holding for a vehiclecomprising an electric power assisted steering by means of a steeringsystem with a steering assistance actuator and one or more controllablevehicle state actuators and comprising an on-board vision system,incorporating the steps of: measurement, with the aid of the on-boardvision system, using one or more sensors of at least one vehicleposition input signal representing one or more vehicle states,determination, in a relative vehicle position calculation function ormeans, from said one or more measured vehicle position input signals ofa relative vehicle lane position in the form of a lane curvature and/ora lane curvature derivative, calculation, in the relative vehicleposition calculation function or means, of a target lateral state vectorconsisting of one or more of the following target values; a target yawand/or lateral vehicle state and a derivative of said target yaw and/orlateral vehicle state, measurement of at least one steering input signalwith the aid of a sensor, determination in a driver torque calculationfunction or means, from said one or more measured steering inputsignals, of a torque value applied by the driver via a steering wheel,wherein method further comprises the steps of: transformation, in atarget relative vehicle state calculation means or function, of saidtorque value applied by the driver to a, relative to the afore-mentionedtarget lateral state vector, target delta lateral state vector,consisting of one or more of the following target delta values; a targetdelta yaw and/or lateral vehicle state and a derivative of said targetdelta yaw and/or lateral vehicle state, adding said target lateral statevector and said target delta lateral state vector together, using a fromthe addition (460) resulting mixed control target lateral state vectoras reference signal to one or more controllers for the control of theone or more vehicle state actuators.
 2. The method according to claim 1,wherein: the target lateral state vector at least comprises a target yawand/or lateral vehicle state, and that said target yaw and/or lateralvehicle state is a target curvature, and in that the target deltalateral state vector at least comprises a target delta yaw and/orlateral vehicle state, and that said target delta yaw and/or lateralvehicle state is a target delta curvature.
 3. The method according toclaim 2, wherein: the target lateral state vector further comprises aderivative of the target yaw and/or lateral vehicle state, and that saidderivative of the target yaw and/or lateral vehicle state is a targetcurvature derivative, and in that the target delta lateral state vectorfurther comprises a derivative of the target delta yaw and/or lateralvehicle state, and that said derivative of the target delta yaw and/orlateral vehicle state is a target delta curvature derivative.
 4. Themethod according to claim 1, wherein from the torque value applied bythe driver via a steering wheel, a compensation torque comprising one ormore of the following compensation torque parts: a tyre friction torque,which is a function of a sensed or estimated yaw and/or lateral vehiclestate, a steering system friction torque, which is a function of asensed or estimated yaw and/or lateral vehicle state, a damping torque,which is a function of a derivative of a sensed or estimated yaw and/orlateral vehicle state, and a returnability torque, which is a functionof a sensed or estimated yaw and/or lateral vehicle state, is/aresubtracted.
 5. The method according to claim 4, wherein the compensationtorque at least comprises the tyre friction torque, and in that thesensed or estimated yaw and/or lateral vehicle state for the input tothe tyre friction torque of the compensation torque part is a sensed orestimated steering angle.
 6. The method according to claim 4, whereinthe compensation torque at least comprises the steering system frictiontorque, and in that the sensed or estimated yaw and/or lateral vehiclestate for the input to the steering system friction torque of thecompensation torque part is a sensed or estimated steering angle.
 7. Themethod according to claim 1 wherein the compensation torque at leastcomprises the damping torque, and in that the derivative of the sensedor estimated yaw and/or lateral vehicle state for the input to thedamping torque of the compensation torque part is a sensed or estimatedsteering angular speed.
 8. The method according to claim 1, wherein thecompensation torque at least comprises the returnability torque, and inthat the sensed or estimated yaw and/or lateral vehicle state for theinput to the returnability torque of the compensation torque part is asensed or estimated steering angle.
 9. The method according to claim 1,wherein the compensation torque at least comprises the tyre frictiontorque, and in that the tyre friction torque, which is a function of asensed or estimated yaw and/or lateral vehicle state, is compensated bysubtracting the target yaw and/or lateral vehicle state from the sensedor estimated yaw and/or lateral vehicle state to a new yaw and/orlateral vehicle state and then to use this new yaw and/or lateralvehicle state (instead of the sensed or estimated yaw and/or lateralvehicle state) as input to the tyre friction torque.
 10. The methodaccording to claim 1, wherein the compensation torque at least comprisesthe steering system friction torque, and in that the steering systemfriction torque, which is a function of a sensed or estimated yaw and/orlateral vehicle state, is compensated by subtracting the target yawand/or lateral vehicle state from the sensed or estimated yaw and/orlateral vehicle state to a new yaw and/or lateral vehicle state and thento use this new yaw and/or lateral vehicle state (instead of the sensedor estimated yaw and/or lateral vehicle state) as input to the steeringsystem friction torque.
 11. The method according to claim 1, wherein thecompensation torque at least comprises the damping torque, and in thatthe damping torque, which is a function of a derivative of a sensed orestimated yaw and/or lateral vehicle state, is compensated bysubtracting the derivative of the target yaw and/or lateral vehiclestate from the derivative of the sensed or estimated yaw and/or lateralvehicle state to a new derivative of a yaw and/or lateral vehicle stateand then to use this new derivative of a yaw and/or lateral vehiclestate (instead of the derivative of the sensed or estimated yaw and/orlateral vehicle state) as input to the damping torque.
 12. The methodaccording to claim 1, wherein the compensation torque at least comprisesthe returnability torque, and in that the returnability torque, which isa function of a sensed or estimated yaw and/or lateral vehicle state, iscompensated by subtracting the target yaw and/or lateral vehicle statefrom the sensed or estimated yaw and/or lateral vehicle state to a newyaw and/or lateral vehicle state and then to use this new yaw and/orlateral vehicle state (instead of the sensed or estimated yaw and/orlateral vehicle state) as input to the returnability torque.
 13. Themethod according to claim 1 wherein the compensation torque at leastcomprises the tyre friction torque, and in that the tyre frictiontorque, which is a function of a sensed or estimated steering angle, iscompensated by subtracting the target steering angle from the sensed orestimated steering angle to a new steering angle and then to use thisnew steering angle (instead of the sensed or estimated steering angle)as input to the tyre friction torque.
 14. The method according to claim1, wherein the compensation torque at least comprises the steeringsystem friction torque, and in that the steering system friction torque,which is a function of a sensed or estimated steering angle, iscompensated by subtracting the target steering angle from the sensed orestimated steering angle to a new steering angle and then to use thisnew steering angle (instead of the sensed or estimated steering angle)as input to the steering system friction torque.
 15. The methodaccording claim 1, wherein the compensation torque at least comprisesthe damping torque, and in that the damping torque, which is a functionof a sensed or estimated steering angular speed, is compensated bysubtracting the target steering angular speed from the sensed orestimated steering angular speed to a new steering angular speed andthen to use this new steering angular speed (instead of the sensed orestimated steering angular speed) as input to the damping torque. 16.The method according claim 1, wherein the compensation torque at leastcomprises the returnability torque, and in that the returnabilitytorque, which is a function of a sensed or estimated steering angle, iscompensated by subtracting the target steering angle from the sensed orestimated steering angle to a new steering angle and then to use thisnew steering angle (instead of the sensed or estimated steering angle)as input to the returnability torque.
 17. A system for controllingvehicle lane holding for a vehicle comprising an electric power assistedsteering by means of a steering system with a steering assistanceactuator and one or more controllable vehicle state actuators andcomprising an on-board vision system, comprising: means for measurement,with the aid of the on-board vision system, using one or more sensors,of at least one vehicle position input signal representing one or morevehicle states, a relative vehicle position calculation function ormeans adapted for determination, from said one or more measured vehicleposition input signals, of a relative vehicle lane position in the formof a lane curvature and/or a lane curvature derivative, and forcalculation, of a target lateral state vector consisting of one or moreof the following target values; a target yaw and/or lateral vehiclestate and a derivative of said target yaw and/or lateral vehicle state,means for measurement of at least one steering input signal with the aidof a sensor, driver torque calculation function or means for, from saidone or more measured steering input signals, determination of a torquevalue applied by the driver via a steering wheel, wherein it furthercomprises: a target relative vehicle state calculation means or functionfor transformation of said torque value applied by the driver to a,relative to the afore-mentioned target lateral state vector, targetdelta lateral state vector, consisting of one or more of the followingtarget delta values; a target delta yaw and/or lateral vehicle state anda derivative of said target delta yaw and/or lateral vehicle state,addition means for adding said target lateral state vector and saidtarget delta lateral state vector together, vehicle state control meansusing a from the addition resulting mixed control target lateral statevector as reference signal to one or more controllers for controllingthe one or more vehicle state actuators.
 18. The system according toclaim 17, wherein the target lateral state vector at least comprises atarget yaw and/or lateral vehicle state, and that said target yaw and/orlateral vehicle state is a target curvature, and the target deltalateral state vector at least comprises a target delta yaw and/orlateral vehicle state, and that said target delta yaw and/or lateralvehicle state is a target delta curvature.
 19. The system according toclaim 18, wherein: the target lateral state vector further comprises aderivative of the target yaw and/or lateral vehicle state, and that saidderivative of the target yaw and/or lateral vehicle state is a targetcurvature derivative, and in that the target delta lateral state vectorfurther comprises a derivative of the target delta yaw and/or lateralvehicle state, and that said derivative of the target delta yaw and/orlateral vehicle state is a target delta curvature derivative.